The strategic imperative for sentinel pathogens in planetary health

Pathogen-by-pathogen monitoring is impractical across the broad, dynamic, and infrastructure-dependent WASH disease landscape. WHO identifies more than 50 organisms transmitted through inadequate WASH and highlights ten that contribute substantially to the burden: pathogenic Escherichia coli, Vibrio cholerae, Salmonella spp., Shigella, Campylobacter, rotavirus, norovirus, hepatitis A virus, adenovirus, and Cryptosporidium [6]. This diversity creates a signal-to-noise problem. Surveillance must identify organisms that not only document contamination but also provide actionable information about system failure.

A useful sentinel pathogen must therefore do more than appear on a priority list. It must connect biological plausibility with operational value. Specifically, it should circulate across the compartments where risks arise; remain detectable under environmentally variable conditions; respond to sanitation failure, hydrological extremes, and antimicrobial selection; carry interpretable genomic and AMR signals; and be measurable using tools that are scalable beyond highly resourced laboratories. Among major WASH-related pathogens, Salmonella meets these requirements unusually well [5–7].

Why Salmonella is a promising sentinel: Five core advantages

1. One Health connectivity. Non-typhoidal Salmonella circulates among livestock, wildlife, food, water, and people. Human-adapted S. Typhi and S. Paratyphi more specifically signal human faecal contamination and unsafe water. Together, they provide a dual lens on zoonotic connectivity and sanitation failure [6–8].
2. Environmental resilience. Salmonella persists in water, soil, sediment, plant surfaces, and food-production environments, and its survival responds to conditions disrupted by heat, floods, and drought [9]. In a study of 488,232 genomes, climate-associated increases in AMR-gene abundance were estimated in 82 of 100 countries evaluated [5].
3. Genomic plasticity. Mobile genetic elements structure the Salmonella resistome. Whole-genome sequencing can therefore help distinguish clonal expansion, repeated introduction, and resistance dissemination, rather than merely record presence or absence [10].
4. WASH actionability. Detection can indicate failures in source protection, treatment, distribution integrity, sanitation containment, irrigation-water quality, or animal-waste management. WHO’s identification of Salmonella as a priority WASH-related pathogen supports its use as a context-specific verification signal within risk-based water safety management [3,6].
5. Clinical relevance and scalability. WHO classifies fluoroquinolone-resistant S. Typhi and non-typhoidal Salmonella as high-priority bacterial pathogens [11]. Typhoidal and invasive non-typhoidal disease remain clinically important, especially in vulnerable populations [12]. Culture and antimicrobial-susceptibility testing are widely established, while referral-based sequencing can add source attribution and transmission tracking [13].

Salmonella should complement, not replace, faecal indicators, sanitary inspection, and pathogen-specific monitoring. No single organism captures every WASH failure or the behaviour of viruses and disinfectant-resistant protozoa.

The proposed SICREW framework: From climate signal to preventive action

SICREW is a four-tier pathway aligned with WHO’s risk-based, catchment-to-consumer approach [3,4]. It links climate warnings to locally interpretable biological signals and predefined public-health responses, as summarized in Table 1, thereby operationalising calls to address climate change and AMR as interconnected issues [14].

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Table 1. The SICREW framework: operational tiers for sentinel-driven surveillance.

https://doi.org/10.1371/journal.pclm.0001040.t001

SICREW would shift Salmonella surveillance from retrospective detection toward anticipatory public health intelligence. This shift is especially relevant for low- and middle-income countries, where broad multiplex pathogen testing may remain financially or technically unrealistic, but Salmonella culture, antimicrobial susceptibility testing, and referral-based sequencing can be built through existing clinical, food safety, and public health laboratory networks, which can also benefit Global Health, including high-income countries.

Implementation priorities for planetary health

Moving SICREW into practice requires four commitments. First, climate triggers and response protocols should be incorporated into climate-resilient water safety planning. Pilots should predefine sampling triggers, measure warning lead time, quantify false alerts, and test whether sentinel signals change WASH or AMR decisions. Second, demonstration projects should focus on climate-vulnerable settings with high enteric-disease burdens, rapid urbanisation, weak sanitation infrastructure, and established or emerging One Health AMR networks. Third, investment should build local capacity in culture, antimicrobial-susceptibility testing, sequencing, bioinformatics, interpretation, and decision-making. Data-sharing arrangements should protect community interests, return actionable results to participating countries, and avoid extractive surveillance. Infrastructural, institutional, and social barriers can otherwise undermine environmental AMR surveillance in low- and middle-income settings [15]. Fourth, utilities and ministries should specify intervention thresholds, responsible actors, financing, and evaluation criteria before thresholds are crossed. This emphasis on accountable, multisectoral implementation is consistent with WHO’s WASH strategy [4].

The evidence base should advance alongside implementation. Priorities include quantifying climate–Salmonella–AMR dose–response relationships and establishing site-specific baselines. Studies should also compare sentinel and syndromic surveillance, evaluate the added value of wastewater and environmental sampling, and test how signals can guide WASH investment, vaccination, food-chain controls, and antimicrobial stewardship [14]. SICREW should be judged by predictive lead time, decision usefulness, cost-effectiveness, and equitable prevention, not by sample volume or dashboard production.

Conclusion: From sentinel to protective action

Climate change is reshaping the environmental conditions under which enteric pathogens persist, move, and acquire resistance. Salmonella offers a practical, complementary sentinel for the climate–AMR–WASH nexus because it connects human disease, animal reservoirs, environmental persistence, genomic change, AMR emergence, and WASH-system performance. SICREW provides a pathway for translating that biological signal into preventive action by integrating climate intelligence, water safety planning, One Health surveillance, and policy feedback. Its utility must be locally validated and governed equitably; nevertheless, it offers a concrete, neat route from fragmented monitoring towards earlier, climate-resilient protection.